A semiconductor device, such as a tunable semiconductor laser or a refractive index control semiconductor optical modulator, which controls light by changing a refractive index by a current/voltage has become important in the field of optical communications. The present invention relates to an optical semiconductor device capable of largely changing a refractive index with a small current injection amount.
Refractive index control optical semiconductor devices of a prior art and the present invention will be described below using a wavelength tuning layer in a tunable semiconductor laser as an example. FIG. 5 shows the sectional structure of a tunable semiconductor laser, and FIG. 6 shows the layer structure of a wavelength tuning layer of the prior art (e.g., Y. Sakata et al., p. 4-202, Proceedings of the IEICE Spring Conference, March, 1993). Referring to FIG. 5, a tunable laser comprises a p-type InP buffer layer 15 stacked on a p-type InP substrate 14, a wavelength tuning layer 16 selectively formed on the p-type InP buffer layer 15, a multilayered structure 13 having a mesa-shaped section and consisting of an n-type InP layer (cladding layer) 17, a multi-quantum well active layer 18 having a bandgap wavelength of 1.55 .mu.m and a semiconductor p-n junction structure for outputting a laser beam, and a p-type InP layer (cladding layer) 19, an n-type InP layer 20 for burying the multilayered structure 13 therein to horizontally and vertically form a double heterojunction on the p-type InP buffer layer 15, a trapezoidal p-type InP layer 21 and a trapezoidal p-type InGaAs layer 22 sequentially formed on the p-type InP layer 19 and the n-type InP layer 20 therearound, an insulating layer 23 formed on the surfaces of the p-type InGaAs layer 22 and the n-type InP layer 20 and on the side surface of the p-type InP layer 21 such that the p-type InGaAs layer 22 and the n-type InP structure 20 are partially exposed, p-type electrodes 24 and 25 formed on the p-type InGaAs layer 22 exposed from the insulating layer 23 and on the lower surface of the p-type InP substrate 14, respectively, and an n-type electrode 26 formed on the n-type InP layer 20 exposed from the insulating layer 23. A diffraction grating 27 for selecting an oscillation wavelength in a laser resonator direction is formed at the interface between the p-type InP buffer layer 15 and the wavelength tuning layer 16. A current is injected into the active layer 18 using the p- and n-type electrodes 24 and 26 to output light. Independently of the current of the active layer, a current is caused to flow in the wavelength tuning layer 16 using the p- and n-type electrodes 25 and 26 so as to inject carriers into the wavelength tuning layer. The wavelength tuning layer 16 serving as a refractive index control optical semiconductor layer has a refractive index which changes due to current injection, and the equivalent refractive index of an optical waveguide constituted by the active layer 18, the n-type InP layer 17, and the wavelength tuning layer 16 changes. As a result, the optical pitch of the diffraction grating 27 changes, and a laser oscillation wavelength changes.
Referring to FIG. 6, the wavelength tuning layer 16 comprises a three-period multi-quantum well layer and an InGaAsP guide layer 31 having a thickness of 200 nm and a bandgap wavelength of 1.29 .mu.m. The three-period multi-quantum well layer is constituted by an InGaAsP guide layer 28 having a thickness of 20 nm and a bandgap wavelength of 1.29 .mu.m, InGaAsP quantum well layers 29 each having a thickness of 10 nm and a bandgap wavelength of 1.45 .mu.m, and InGaAsP barrier layers 30 each having a thickness of 20 nm and a bandgap wavelength of 1.29 .mu.m. All the semiconductor layers are lattice-matched with the p-type InP substrate 14. A refractive index change of the wavelength tuning layer 16 caused by carrier injection occurs due to an anomalous dispersion change caused by band filling of the InGaAsP quantum well layers 29 and a plasma dispersion change of carriers of the entire area of the wavelength tuning layer 16. However, the contributions of the two dispersion components to the wavelength tuning layer 16 are equal to each other. When such a wavelength tuning layer is used, a wavelength change of about 3 nm can be obtained with a wavelength tuning current of 0 to 50 mA.
In the prior art, main factors for limiting a wavelength tuning amount are heat generation of an element caused by a wavelength tuning current and a laser optical loss caused by inter-valence band absorption in the wavelength tuning layer. The former increases with an increase in wavelength tuning current, and the latter increases with an increase in carrier density of the wavelength tuning layer. As a result, a laser output decreases with an increase in tuning current. In addition, although a refractive index change of the wavelength tuning layer depends on the carrier density of the wavelength tuning layer, the relationship between a current density (J) and the carrier density (N) is given by J=eN/.tau., and the current density considerably depends on a carrier lifetime .tau.. In this case, e represents an elementary electric charge. Since the carrier lifetime decreases with an increase in carrier density, a current required for increasing a unit carrier density increases with an increase in carrier density. Therefore, the maximum amount of a wavelength tuning current with which a practical laser output can be obtained is about 50 mA.